Like many mechanical devices marine controllable pitch propellers ("CPP's") require periodic preventive maintenance and rehabilitation to replace wearing parts for assurance against breakdown or less than optimum operation. The CPP is removed from the vessel and disassembled, and various components, such as seals and bearings, are replaced.
In some designs of CPP's, for example, many CPP's built by Bird-Johnson Company of Walpole, Mass., which are in widespread use, the blades are attached to spindles that rotate for pitch adjustment in blade-mounting port openings in the propeller hub body. The blade-mounting assembly for each blade includes an externally threaded bearing ring that is threaded into internal threads in the port opening. For assurance against unthreading of the bearing ring, several holes are drilled into the hub body and bearing ring at their juncture, and dowels are installed in the holes to lock the bearing ring in place. The port opening also has a seating surface adjacent the bearing ring for an O-ring seal.
At intervals of about five or six years the maintenance procedures for such CPP's call for replacement of the bearing rings, which are, of course, wearing parts. Often, the hub body requires only minor repairs when the sealing rings are replaced. Even though new holes have to be drilled for the locking dowels for the replacement bearing rings, because it is not feasible to reuse the parts of the holes in the hub body for the new dowels, the portions of the dowel holes in the hub body need not be filled.
For a variety of reasons, such as loss of integrity of the threads in the hub body due to too many unused dowel holes from previous reconditionings of the CPP or damaged threads, it is occasionally necessary to recondition one or more of the port openings in the hub body. The threads and the O-ring seating area of the port opening are machined to expose clean base material. Any casting defects in the exposed material and the dowel holes are ground or otherwise excavated. After cleaning to remove any oil residue and oxides in the work area, the excavated areas and the entire thread area and the O-ring seating area of the port opening are built up to restore material for the threads and the proper O-ring seating surface by applying weld filler material. The application of the weld filler material has been done by hand and has required considerable time and great care to avoid thermal stress-cracking and thermal distortion of the hub at the welding site. The hand-welding operation on each hub port may take as long as one week, and sometimes longer, to complete. More often than not, efforts to restore the threaded area with weld metal are unsuccessful because of cracking or distortion.